A thermoplastic polyester series resin (hereinafter, is referred to as PAT) is known as an engineering plastic and has excellent properties which are lacking in polystyrene and polyethylene. For example, PAT has a large rigidity, a good form stability, and an excellent heat resistance of enduring 200.degree. C. Thus, it was tried to make tough heat-insulating materials, cushioning materials, package containers, food containers, etc., having good heat resistance by foaming PAT. However, different from polystyrene and polyethylene, it was not easy to foam PAT. Even when PAT can be foamed, the foamed material has a low expansion ratio only.
The reason that PAT is reluctant to be foamed is by the crystallinity of PAT and is that it is difficult to keep the viscosity of PAT suitable for foaming when PAT is melted. More specifically, when PAT is heated, PAT is suddenly softened at a certain temperature to become a liquid having a low viscosity. That is, for PAT , the temperatures suitable for foaming are limited to a very narrow temperature range and, hence, it is difficult to keep PAT at a temperature suitable for foaming. Also, when PAT becomes a liquid having a low viscosity, a gas acting as a foaming agent is immediately released therefrom. Therefore, even when PAT can be foamed, the expansion ratio is very low and it is difficult to make foamed materials of PAT of low density foamed at a high expansion ratio.
For solving these problems, various attempts have been made. For example, it was proposed to mix an epoxy compound with PAT or mix a metal belonging to group Ia or group IIa of the periodic table with PAT. However, by these proposals, large-sized foamed materials foamed at a high expansion ratio could not be obtained.
Also, JP-A-59-135237 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 4,462,947 propose the use of high-molecular weight chain aromatic polycarbonate as a foaming agent for obtaining PAT foamed materials. However, by these techniques, foamed materials having a high expansion ratio cannot be obtained. In fact, in the above-cited patents, as an example of the foamed material obtained, only a material having a density of about 0.83 g/cm.sup.3 is described.
As other attempt, JP-A-55-2045 describes a method of obtaining a foamed material having a low density by introducing high-temperature PAT containing a foaming agent directly after being extruded from an extruding machine into a reduced pressure zone and foaming it under reduced pressure. However, since high-temperature PAT containing a foaming agent generally has uneven surfaces by foaming and has an irregular external form, it is not easy to construct a reduced pressure zone capable of continuously passing such high-temperature PAT. In particular, it is difficult to make sufficiently reduced-pressure sealing of the outlet of the foamed material. Accordingly, the aforesaid method has a disadvantage of being not easily practiced.